Clinically significant weight loss (5-10%) is associated with cardiovascular health benefits for obese adults, and participation in adequate amounts of physical activity is a critical factor for weight maintenance after weight loss. However, the recommended amount of physical activity needed to promote weight maintenance is based primarily on retrospective studies that quantified physical activity levels through questionnaires which tend to overestimate physical activity levels. In addition, the present literature has provided little data on impact of these physical activity levels on cardiovascular risk factors, which may have equal or more clinical importance compared to weight changes. The purpose of the Prescribed Exercise to Reduce Recidivism After Weight Loss (PREVAIL) study is to evaluate the effect aerobic exercise training amount on weight maintenance following clinically significant weight loss in obese men and women (BMI 35-45) ages 18-65 years old. Volunteers will complete an OPTIFAST weight loss program at VIDANT Wellness Nutrition Clinic. Individuals who achieve a weight loss of at least 7% from baseline weight will be subsequently randomized to levels of aerobic training approximately half of PA recommendations (B-REC), consistent with physical activity recommendations (PA- REC), or weight maintenance recommendations (WM-REC) for 9 additional months. Exercise sessions will be directly supervised by study staff. Non-exercise physical activity will be captured through accelerometry during the entire intervention in all randomization groups and total energy expenditure will be assessed by doubly labeled water (the gold standard technique). The primary outcome of the PREVAIL study will be change in weight from the completion of OPTIFAST program (achievement of clinically significant weight loss) to the end of exercise training. Main secondary measures include proportion of participants achieving weight maintenance, changes in clinically relevant cardiometabolic risk factors (e.g. fat mass, lipids, c-reactive protein, fitness and insulin sensitivity, pulsewave velocity, augmentation index and lipoprotein particle size, and changes in quality of life) between study groups. The present study addresses a major need for research identified by both the American College of Sports Medicine and a recent NHLBI sponsored working group on weight maintenance.